small improvment to placeholder html

verse/themes/editor/placeholder.html

@@ -1,15 +1,23 @@
-<pre>Write some test HTML here</pre>
-<div id="folder" class="dir"><a href='example'>example folder</a></div>
+<h2>Write some test HTML here</h2>
+
+<div id="folder" class="dir">
+  <a href='example'>example folder</a>
+</div>
+
 <div id="willemdekooning-logo" class="png">
   <figure>
     <img class="image" src="https://www.wdka.nl/build/img/willemdekooning-logo.png">
     <figcaption>logo.png</figcaption>
   </figure>
 </div>
+
 <div id="example_video" class="mp4">
   <video controls>
     <source src="example_video.mp4">
   </video>
   <span class="filename">example_video.mp4</span>
 </div>
-<div id="unknown.file" class="plain unkown-file"><a href='unknown.file'>unknown.file</a></div>
+
+<div id="unknown.file" class="plain unkown-file">
+  <a href='unknown.file'>unknown.file</a>
+</div>